1. Field of the Invention
The present invention relates to a hydraulic control system with a one-way orifice for automatic transmissions.
2. Description of Related Art
One of the conventionally proposed hydraulic control systems for automatic transmissions is disclosed in JP-U 63-101355. In this hydraulic control system, interposed between two valve bodies which constitute a hydraulic circuit when connected to each other is a separate plate which is formed with a one-way orifice of a small-diameter hole having one end or both ends enlarged in a conical surface, or a one-way orifice of small-diameter holes so as to reduce noise of oil passing through the one-way orifice.
With the known hydraulic control system, oil noise is reduced by the conical surface of the one-way orifice. However, the magnitude of oil noise is dependent on the flow rate and the viscosity of oil passing through the one-way orifice, so that, in order to obtain sufficient noise eliminating effect, the angle of the conical surface of the one-way orifice or the length of a non-conical surface portion thereof, i.e., the length of contact of oil with the one-way orifice should be changed in accordance with the flow rate and the viscosity of oil, resulting in a problem of increased kinds of separate plates.
A reduction in the length of contact of oil is obtained by a method of increasing the depth of the conical surface without changing the angle of the conical surface, or a method of increasing the depth of the conical surface by decreasing the angle of the conical surface without changing the radial dimension of a large-diameter end thereof. However, according to the former method, a large-diameter end of the conical surface has increased radial dimension, producing possible interference with a sealing member of the valve body. On the other hand, according to the latter method, decreased angle of the conical surface causes a reduction in noise eliminating effect.